Method of testing and test set



Oct. '29, 1929. J. w. DEHN nsmon OF TESTING AND TEST 551' Filed May 51, 1927 llHilo fi Z/VVENTUR JOJEAH MOM/v Arm/ms) Patented Oct. 29, 1929 UNITED STATES PATENT. OFFICE I JOSEPH wnnnn, or BROOKLYN, NEW YORK, nssrerron 'ro wns'rmuv nrincrmccom I PANY, mconronnrnn, ornnw YORK, N. Y.', A CORPORATION on NEW YORK METHOD or mns'rrnc arm mns'r SET ,Applicationfiled May 31, 1927. SeriaI No. 195,225.

This inventionrelatesto a-method and to apparatus -for measuring an electrical constant of a transmission line without materially disturbing the normal functions of such line.

lvlore specifically, it relates to the measuring of the electrical impedance of a battery feed in' transmission lineextending from a main exchange to a private branch exchange in a telephonesystem and without interrupting the operation of the private branch exchange.

It is necessary in the maintenance of tele phone service where a large number ofprivate branch exchanges are worked from a central orlice to'check up periodically on the re istance of the battery feedingtransmissien linesextending from the main exchange to the private branch exchanges. Heretotore, a number of methods of measuring such resistances have been employed but in so far the applicant is aware all oft he relatively accuratemethods have involved disconnect ing the feeder from the private branch exchange. Disconnecting the feeder has'necessitated taking the measurement at some time when the private branch exchange was not in. operation. It was therefore not always convenient to take the measurement, particularly as some of the private branch exchanges are in operation during the major portion of the twenty four hoursof the day.

The present invention provides a method involving two steps in which knownmethods of resistance measurement;arefapplied in a particular manner and sequence to obtain the desired measurement. i i

In accordance, with the invention, the first step consists "in connecting in parallel with the load at the private branch exchange an impedance which is equal to or bears a 'de finite ratio to the load of the private branch I exchange. This step may be conveniently per-formed using a W'heatstone bridge ar raugemen't. The only precaution necessary to observe is for the tester to make sure that the load on the private branch exchange has not changed during the measuring interval by'some. act of the operator such as the changing of a cord.

" Having connected such impedance par allel Withthe private branch exchange load,

the potential at the point of connectio of e .2

load. and parallel impedance to the line mayv be noted. The parallel impedance is then disconnected and an impedance is connected in series with the load and is adjusted until the potential at the point of connection of the load with the feeder is the, same as in the previous step. Itcan be shown thatunder the conditions outlined for the measurement,

the resistance necessary to bring this point tothe same potential as before, IS a direct measure of the impedance of the feeder.

In accordance with the invention, apparatus for performing the method as above outlined is provided in the convenient and compact form of a single instrun'ient. The tester can make the'entire measurement by merely manipulating the keys of the instrus ment in sequence or alternately, noting the eiiect in a telephone receiver. This embodis ment or the invention, comprises adjustable resistance, two condensers and a telephone receiver connected into keyed circuits and arranged to compare the potentials at a point between the line and the load under different c1rcu1t conditions. The apparatus may conveniently be arranged in the form of a port-' be outlined with particular reference to Fig. l of the drawing. In this figure the battery P is located at the main office and the load at the private'branch exchange is indicated by theresistance L. This load is'made up or the apparatus in use at the private branch exchange which varies fromtime to time,

but which will ordinarily remain constant :tor .a sufiicient time to permitjthenecessary measurements to be made. The transmission line which isknown in the art as a'private branch exchange battery feeder extends from theabattery P to the terminal K at the private branch exchange. While the resistance of he iieederis in practice distributed along its length it has for convenience of description been shown as a lumped resistance F which represents the resistance of the feeder as a whole, in othe words the resistance to be measured. The point K is ordinarily connected to the'point J, by a fuse indicated in dotted lines. The adjustable resistances A and B "form a part of the test set which is connected across the points K and J and the ituse is thenremoved. 7

v The first step in the method of determining the resistance of F is to adjust A to a definite ratio (usually equality) to L holding B shortcircuited. The potential at the point J with A connected to ground may then be determined. i r H The second step consists in disconnecting A from ground, removing the short-circuit from B and adjusting the resistance B until the potential at J is the same as that previously noted; When this condition is obtained F=nB where n is the ratio of A to L. If n is' unity F=B; .This may be shown mathematically as follows:

If the potential at J or E is to be the same under the two conditions set forth above and E the potential of the battery P remains constant it follows that. is the sameunder both test conditions.

lVith B short circuited and A included AL+AF+FLVZL+B+F VAL+AB+AF=AL+AF+FL AB=FL A If It 71 1, F=B

is assumed to be a, F=nB In 2 the circuit arrangement used in a testset embodying the invention is particul rly designed for measurement of the resist- -.nce of the private branch exchange battery ieeders butis not limited to such use. In'the test-set as actually constructed the various resistances, keys and. circuit connections are mounted in a portable box and the adjustable resistances take the form of ordinary dial type resistances such as are commonly used in ordinary Wheatst-one bridge measuring devices. The set is provided with the usual binding posts for connection to the feeder to be measured and to theload of the private branch exchange. In Fig. 2 the same reference characters have been used to designate the parts which are common to that figure and to Fig. 1. The additional apparatus not shown in Fig. 1 will be now described.

There are two keys used, one of which is designated as BF and the other as AL. These keys are 01 the ordinary type used in telephone exchanges in which the contacts are operated by an oscillating lever. The key AL is arranged so that when its lever is thrown in one direction the contacts are in the position shown and when thrown in the other direction the contacts are moved to take up alternate or opposite positions.

The BF key contains two sets of contacts which are operated from a single oscillating lever. The contacts are shown in the neutral position. On movement from neutralrposition to one position, the upper set of contacts, as shown, are actuated, and on movement to the other position, the lower set of contacts are actuated. The arrangement is such that the operating lever BF in passing from one extreme position to the other causes the res toration of one set of contacts and changes the setting of the other contacts. It may also be mentioned that the contact 1 of the upper set of contacts connected with the BF key is arranged so that it' is opened before the other contacts are closed and the key is returned to normal.

An inspection of the circuitswill show that with the BF key in its upper position the condenser Z will be charged at a point J. hen the BF key is in upper or in its normal or neutral position the condenser Y is connected to point J through a contact of the AL key in its normal position. When the BF key is in its lower, position the condenser Y is connected to the condenser Z in opposing relation through the telephone receiver R. With the BF key in its normal or neutral position the operation of the AL key merely places the telephone receiver across 1 the opposite points of the Wheatstone bridge formed by arms C, D, L and A.

twill now be assumed that it is desired to measure'the resistance F with thetest set of Fig. 2 connected as shown, and the operation of making this measurement will now be described. With the ratio of C to D at equal'ty, the first step is to make the resistance A equal to the resistance L. It there is a diiierenceof potential between points J and H of the bridge a click will be heard in the telephone as the key A i moved. The A key successively operated. while the r sistance A is adjusted until no click is heard :1, ceases in the telephone receiver R which indicates that there is no diiierenee in potential between the points J and H. As is usual in iVheatstone bridge measurements the ratio on arms C and I) may beequal or bear any definite ratio, but they may Conveniently be equal resistances possibly of low value. It

will be noted that Whilethis measurement is 1 the BF key the manipulation of this key causes the following operations to takeplace.

First, when the key BF is in its upper position condenser Z is charged at point J through contact 1. The key BF opens the contact 1 before it closes the circuit to the A resistance and before it short-circuits the B resistance, so that the charge on thereondenser will not be changed by the closing of these circuits. In the neutral position of the key the condenserYis charged to the potential of point J through an obvious circuit including the contact o'tthe key AL.' The key BF at its lower positionor setting connects the two condensers in series with the telephone It, the charges on the condensers opposing each other. It the potential to which the condensers Y and Z have been charged is equal, no clickwillbe' heard in the receiver This condition indicates that the resistanco has been adjustedto the final desired 'value'and is a measure of the resistance F. The test set shown and described is arranged tocheck the potential of point J for the two circuit conditions, to determine if point Whas the same potential for both conditions." I I For the arrangement of Fig. 2 the equationsare as follows: j f p With 13 short circuited and A included which the same result as obtained. for the circuit of Fig. 1. l I

' Th keysAL and BF may be, operated alternatelyand With considerable rapidity so that eachadjustment and the'tina'l measurement may be made in a very short time. The nieasuredva'lue is readily checked by manipulation, first of key AL and later of keyBF, While listening in the telephone receiye'rIR to make sure there is no click. This check is testing. If the load L is changed in the in-.

terval of taking the measurement the value of resistance A is the only "value that needs to bechanged. v p

vZ-llthoughthe arrangement is particularly adapted to measuring the resistance of pri: vate branchexchange battery feeders by the method above set forth it is also capable of measuring other resistances. 'In order to more readily adapt it for this latter purpose there isprovided a key G and a fixed resistance M which may bepart of the setand may be used as asubstitute for the load L. The resistance F maybe measured independently otthe load or at a point intermediate the battery supply and the normal load. *"It will be obvious that a voltmeter may be substitutedfor each of the condensers Y+Z and the comparison of potentials made directly Alternatively asingle voltmeter may be used and the potentials at point J maybe measuredsuccessively for the two unbalanced ew eeren irriw F ar ies. (and fin: included conditions, the one voltmeter being used for both measurements. k s

It will also be, obvious thatfthe. impedance B may be made up of a variable resistance and a Variable inductance in series with one another, and these may be adjusted to be equal toor to be a measure. of the value F. An A. C. voltmeter or meters may then be used as suggested in the paragraph above in connection with this arrangement when the current in the private branch exchange feeder is an alternating current. The impedance in the two different unbalanced conditions may be then measured directly and successively. The value of impedance B will be properly adjusted when the voltmeter readings are alike for the two difierent unbalanced .conditions.

Itwill be obvious that the method and ap paratus disclosed is available to the measurement of resistances or impedances of any sort, whether the impedance is due to resistance, capacity or inductance, and without departing from the spirit and scope of this in vention.

What is claimed is:

1. The method of measuring an electrical constant of a transmission line without dis- .turbing the load thereon which consists in connecting an impedance in parallel with the load, said impedance being equal to that of the load, determining the potential at the ing the potential at said point, restoring the impedance beyond said point to its original value, and inserting between said source and point an impedance suffic1ent to bring the potential'at the point to the value previously determined, whereby said last mentioned impedance 1s a measure of the impedance to be measured. I

3. The method of measuring the impedance of a transmi-ssion line between a source of energy applied at one end of the line and any point on the line which consists in reducing 7 the impedance beyond said point to one'halt determining the potential atsaid point, restoring the impedance beyond said point to its original value, and inserting between said source and point an impedance suificient to bring the potentialat the point to the value previously determined, whereby said last mentioned impedance equals the impedance a second variable impedance: in parallel with the load beyond the point, varying the second impedance to a predetermined factor of the load, obtaining a measure of the potential at the point, disconnecting the second impedance and the shunt on the first variable impedance, and adjusting the first variable impedance until the measure 01'': the potential at the point is the same as that previously obtained, whereby said first impedance multiplied by a factor determined by the relative values of the second variable impedance and the load beyond the point is equal to the impedance to be measured.

5. The method of measuring the impedance of a transmission line between a source of energy and a point, which consists in connecting a variable impedance to the point and in parallel with a load beyond the point, adjusting the variable impedance to a predetermined ratio of the load; storing a measure of the potential at the point, disconnecting the variable impedance, inserting between the source and point a second variableimpedance, and varying the said second impedance until the measure of the potential at the point is equivalent to the previously stored measure of the potential at the point, whereby the secondimpedance multiplied by the ratio of the first variable impedance to the load impedance equals the impedance to be measured.

6. The method of measuring the resistance of a transmission line between a source of energy and a point, which consists in connectinga variable resistance to the point and in parallel with a load beyond the point, adjusting the variable resistance to a predetermined ratio of theload; storing a measure of the potential at the point, disconnecting the variable resistance, .inserting between the of energy and any pointwhich consists in changing the impedance beyond said point, storing a measure of the potential at the point, restoring the impedance'beyond said point to its original value, inserting between said source and point a variable impedance, storing a measuring of the potential at the point and comparing said last mentioned stored measure of thepotential with the previously stored measure of potential to determine if they are equal,'and alternately varying'the last impedanceand comparing the stored potentials at the point for the two conditions until it is found that the potentials areequal,

whereby the last impedance is a measure of the value of the impedance of the line.

8. The method of measuring an electrical constant of a transmission line without disturbing the load thereon which consists. in connecting an impedance in parallel with the load, said impedance being in a predetermined ratio to that of the load, determining the potential at the junction of the load and the line, removing the first impedance from association with the line and load, and inserting suflicient impedance in series with the load I to establish the same potential at the junction as was previously determined, whereby the last impedance so added multiplied by the ratio of the first variable impedance to the load impedance is equal to said constant.

9. In an impedance measuring set, a plurality of condensers, an indicating device, switching means for, connecting the condensers in succession to a point in a line the impedence of which is to be measured and then to said indicating device, and means for discharging the condensers in opposing relation through the indicating device. I

10. In an impedance measuring devicefor measuring the impedance of a particular part of a line, to a given point, the combination of impedances with switching means and circuits for alternately changing the impedance of the line beyond said point by a given multiple, and for substituting such an im pedance on the other side of said point as to maintain the potential at said point, unchanged.

11. In an impedance measuring set, means for multiplying the impedance on one side of a point in a transmission line by a predetermined value, means for determining the potential at the point, means for restoring the original impedance beyond the point and means for adding sutficient impedance on the other side of said point to bring the potential at said point to the value already determined, said added impedance serving as a measure of the impedance of the transmission line to the said point.

12. In an impedance measuring set, means for multiplying the impedance on oneside of a point in a transmission line, means for storing a measure of the potential at the point under varying conditions, means for restoring the original impedance beyond the point, means for adding impedance on the other side of said point in'order to bring the potential at such point to its former value, means for storing a measure of potential at the point for varying added impedances and means for comparing the stored measures of potential at the point.

13. A resistance measuring device for measuring the resistance of battery feeding lines between telephone exchanges comprising means for alternately connecting a resistance in parallel with the load supplied by said feeding line and connecting an adjusta- I ble resistance in series with said load, and

twofixed ratio arms and" ceiver across said bridge to test the balance thereof, two condensers each connected on one side to ground, a second switch for grounding the variable arm, for shunting or for connecting the variable impedance in series with the bridge and between the point of connection of the bridge. and the line of the private, branch exchange, and for charging one con denser, and a third switch controlled by the key of said second switch and arranged to charge the other condenser at the potential of the point in the line or when operated by the key of said second switch to discharge both condenser-gin opposition, to saidreceiver. 1

15. In a test set, a WVheatstone bridge, means for connecting one end of the bridge to a point in a line to be tested, means for connecting the other end to ground, means to disconnect one arm of said bridge and to substitute therefor a load such as that of a private branch exchange in parallel with the variable arm of the. bridge, a shunted variable impedance connected in serieswith the bridge and with the load of the private branch exchange, but on the other side of the point of connection to the private branch exchange, a receiver, two condensers each connected on one side to ground, a switch operable to connect said receiver acrosssaid bridge to test the balance thereof, and to charge the other condenser at the potential of the point in the line when the variable impedance is shunted, a switch arranged to take the shunt from said impedance, to remove the variable arm from the Wheatstone bridge and to close a path to charge one condenser at the potential of the point of connection of the private branch exchange load, and a third switch controlled by the key of the second switch for connecting the other sides of the condensers simultaneously toopposite sides of the receiver. I

In witness whereof, I'hereunto subscribe my name this 27th day of May, A. D. 1927.

JOSEPH W. DEHN. 

